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n and Properties of Multiple osephson Tunnel Juncti

R. Monaco, A. Polcari, Lucia Capogna

Published Jan 1, 1996• 0 citations
OpenAlex

Abstract

Absbuct- Vertically stacked Josephson tunnel junctions with a high number of junctions (up to n = 20) were realized using a fabrication process based on the NbIAl technology. An experimental study aimed toward their potential applications was made. First, we report on a series of dc measurements, with the main purpose of studying the quality of the I-V curve as a function of the number of barriers in the stack; we found that the junction quality slowly becomes worse as we increase this number. In fact, some thermal effects appear, which result in an increase of the subgap current and a decrease of the width of the current jump at the total gap voltage. Further, we found a low frequency voltage noise on devices with n > 10 that depends Roberto Monaco, Albino Polcari, and Lucia Capogna on the junction area and on the substrate nature. Later, we discuss some RF properties of stacked junctions irradiated with a microwave signal. These measurements show that they seem to behave independently. Once again, the thermal effects drastically influence the response to an external RF drive and seem to limit the use for applications of these devices.

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Absbuct- Vertically stacked Josephson tunnel junctions with a high number of junctions (up to n = 20) were realized using a fabrication process based on the NbIAl technology. An experimental study aimed toward their potential applications was made. First, we report on a series of dc measurements, with the main purpose of studying the quality of the I-V curve as a function of the number of barriers in the stack; we found that the junction quality slowly becomes worse as we increase this number. In fact, some thermal effects appear, which result in an increase of the subgap current and a decrease of the width of the current jump at the total gap voltage. Further, we found a low frequency voltage noise on devices with n > 10 that depends Roberto Monaco, Albino Polcari, and Lucia Capogna on the junction area and on the substrate nature. Later, we discuss some RF properties of stacked junctions irradiated with a microwave signal. These measurements show that they seem to behave independently. Once again, the thermal effects drastically influence the response to an external RF drive and seem to limit the use for applications of these devices.
Stacked Josephson junction arrays located at the rf current antinodes of a superconducting microstripline resonator were investigated. The circuits were designed to obtain highly accurate voltages at cm-waveband microwave driving and low rf-power level. Nb-Al/AlO/sub x/-Nb Josephson junctions with tunneling area of 60 /spl mu/m/spl times/300 /spl mu/m and critical current density up to 7 A/cm/sup 2/ were used. We compared the average voltages of the rf induced maximal current steps in arrays with up to three junctions per stack. Quantized current steps up to 300 mV were observed under microwave irradiation at frequency of 19.20 GHz in circuits with 384 three-junction stacks. The dynamics of stacked tunnel junctions in the presence of rf-currents will be discussed with respect to application as voltage standards.
Abstract In this work, we present a detailed study of the electrical properties of stacked NbN/(TaN/NbN) N Josephson junctions. Cross-sectional scanning transmission electron microscopy analysis of the 5-stacked junction shows that the multilayer interface is very flat, each barrier has the same thickness, and the sidewalls of the junctions are nearly perpendicular to the substrate. Stacked junctions of different sizes and stacking numbers all have only one transition in their current–voltage curves. This indicates that the critical currents of the junctions in the stacked junctions are almost the same, showing the stability and repeatability of the multilayer fabrication and etching process. At 4.2 K, the 4-stacked junction shows excellent Josephson properties with characteristic voltage V c of 3.54 mV, which is about four times the 0.88 mV of the single junction. The temperature dependence of critical current density J c and V c of the stacked junction with N = 1, 2, 4 were measured, all of which can be fitted with dirty-limit theory. Stacked junctions with larger V c or more stacked layers can be achieved by optimizing electrode and barrier thickness, barrier resistivity, and thermal relaxation rate, etc.
Double-barrier niobium tunnel junctions with the middle electrode thinner than the London penetration depth were fabricated. Their I-V curves, annealing behavior, response to external magnetic field, and response to 230 and 350 GHz irradiation in mixer experiments is discussed. Junctions with an integrated tuning circuit designed for the 230 GHz frequency range gave double-sideband receiver noise temperatures of 100 K. Vertically stack arrays can be attractive for various applications. Compared to single-barrier devices possible drawbacks were noted. The I-V curve indicates heating and/or nonequilibrium effects. Double-barrier devices became nonuniform after thermal annealing at 200 °C. The response to the external magnetic field is more complex and considerably larger flux densities are needed to suppress Josephson effects.
High quality NbN-NbOX-Pb Josephson devices have been fabricated by r.f. diode sputtering. The critical temperatures of the NbN films were as high as 16.8 K. The junction geometry was defined by photolithography and the oxide barrier was obtained by thermal oxidation or by a plasma oxidation technique. Very low leakage currents have been obtained, corresponding to values of the parameter Vm = 80 mV (Vm = IO× 2 mV/I (2mV)). The junction microwave quality factor Q and other a.c. properties have been measured in the frequency range 20 - 300 GHz by looking at the devices self-resonant modes. Q values up to 120 at 4.2 K have been found in the low frequency range, indicating potential performances of these devices for microwave applications such as Josephson voltage standards. Theoretical analysis of the results shows that the main contribution to the losses is related to the NbN films surface impedance.
We investigated the behavior of vertically stacked tunnel junctions in a circuit suitable for voltage standards. Such three-dimensional (3D) circuits might offer a considerable reduction of chip size when compared with conventional, two-dimensional circuits. Up to 84 series connected stacks with four junctions in a stack were incorporated into a microstripline. In the current-voltage characteristic, we measured quantized current steps until 100 mV upon applying microwave irradiation in the W band. We observed both chaotic and nonchaotic behavior. Whether chaos or nonchaotic behavior is observed under microwave radiation depends on the coupling strength which is defined by the intermediate niobium layer thickness between the junctions of a stack. Large thickness corresponds to weak coupling and absence of chaos. We showed that 3D circuits are promising for voltage standard application.
Unexpected partly dramatic temperature dependences were registered for the basic tunnel parameters used to characterize a Josephson junction. Measurements of the magnetic field variation of the microwave-power reflection, i.e., a mapping of the Josephson plasma resonance, enabled determinations of the rf small-voltage resistance RJ, the cosφ amplitude ζ, and the effective junction capacitance C in both Pb-O-Pb and PbTl-O-Pb junctions. The uncertainties in the parameter values are large, particularly in the low temperature values of ζ, for which we confirm earlier estimates of ζ, but as the superconducting transition temperature Tc is approached, ζ increases rapidly and goes through a positive maximum close to +1 before it decreases towards zero at Tc. Also, RJ displays a pronounced maximum in the same temperature interval; it decreases rapidly within a narrow range below the temperature of the maximum before it again increases slowly well below Tc. The location in temperature for the two maxima depends upon the barrier via the critical current density, but seems to be material independent. Well below Tc, the ratios of RJ to the normal-state tunnel resistance are also affected by the barrier transmission. These latter properties seem hard to explain within a model based only on the relaxation of the order parameters in the superconductors. A considerable decrease in C close to Tc can be explained by temperature-dependent surface losses in the junction electrodes. The Q-value associated with these could be determined as a function of temperature. In accordance with previous measurements, the Josephson current IJ is reduced at low temperature in small high-resistance tunnel junctions. Smaller deviations in IJ from theoretical estimates can be explained by strong coupling effects.